Epitaxial silicon wafer

ABSTRACT

An epitaxial silicon wafer is provided with a boron-doped silicon substrate and an epitaxial layer formed on a surface of the silicon substrate, wherein the boron concentration in the silicon substrate is 2.7×10 17  atoms/cm 3  or more and 1.3×10 19  atoms/cm 3  or less, and an initial oxygen concentration in the silicon substrate is 11×10 17  atoms/cm 3  or less. When an oxygen precipitate evaluation heat treatment, such as a heat treatment at 700° C. for 3 hours and a heat treatment at 1,000° C. for 16 hours is executed on the epitaxial silicon wafer, the density of oxygen precipitate in the silicon substrate is 1×10 10 /cm 3  or less.

TECHNICAL FIELD

The present invention relates to an epitaxial silicon wafer, and moreparticularly to an epitaxial silicon wafer having an epitaxial layerformed on the surface of a boron-doped p-type silicon substrate.

BACKGROUND ART

Epitaxial silicon wafers are being widely used as substrate material forsemiconductor devices. An epitaxial silicon wafer has an epitaxial layerformed on the surface of a bulk silicon substrate. Since epitaxialsilicon wafers show a high degree of crystal perfection, they can servefor manufacturing of highly reliable, high-quality semiconductordevices.

Silicon substrates containing a p-type or n-type impurity at highconcentration are employed for epitaxial silicon wafers, whoseapplications include solid-state imaging elements and powersemiconductor devices. For example, Patent Literature 1 describes anepitaxial silicon wafer comprising a wafer that shows a nitrogenconcentration of 1×10¹² atoms/cm³ or more or whose electricalresistivity is set to 20 mΩ·cm or less by means of boron-doping and anepitaxial layer arranged on the surface of the wafer, wherein theinitial oxygen concentration of the epitaxial silicon wafer is 14×10¹⁷atoms/cm³ or less.

If compared with ordinary wafers, silicon wafers that are heavily dopedwith nitrogen or boron can easily produce oxygen precipitates in devicemanufacturing processes because nitrogen and boron raise the stabilityof oxygen precipitation cores. Therefore, if laser spike annealing (LSA)is executed in a state where plate-shaped oxygen precipitates has grownto show a large size, dislocation can easily take place, starting fromsuch an oxygen precipitation core. However, an epitaxial silicon waferdescribed above can prevent dislocation that starts from an oxygenprecipitation core from taking place even when LSA is executed in thedevice fabrication process.

Patent Literature 2 describes an epitaxial wafer to be used for abackside illumination type solid-state imaging element, the epitaxialsilicon wafer comprising a p-type silicon substrate containing carbonand nitrogen added thereto and having an electrical resistivity of lessthan 100 mΩ·cm, a p-type first epitaxial layer formed on the p-typesilicon substrate and a p-type or n-type second epitaxial layer formedon the p-type first epitaxial layer, the interstitial oxygenconcentration in the p-type silicon substrate being between 10×10¹⁷atoms/cm³ and 20×10¹⁷ atoms/cm³, the density of precipitate in a centralportion of the p-type silicon substrate as viewed in the depth directionthereof being 5×10⁵ cm² or more and 5×10⁷ cm² or less. According to thisepitaxial wafer, Backside illumination type solid-state imaging elementscan be manufactured at high yield.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent Application Laid-Open No.2011-228459

[Patent Literature 2] Japanese Patent Application Laid-Open No.2012-138576

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

With regard to epitaxial wafers to be used for solid-state imagingelements and other applications, the boron contained in the siliconsubstrate is indispensable to secure the gettering ability and lower theelectric resistance of the silicon substrate. However, on the otherhand, when boron is diffused to a large extent from the siliconsubstrate toward the epitaxial layer side, there arises a problem thatthe impurity profile of the epitaxial layer is changed to degrade theuniformity of in-plane electrical resistivity of the wafer.Additionally, the width of the transition region of boron concentration(resistance-varying layer) in the epitaxial layer at and near theboundary of the silicon substrate and the epitaxial layer is increasedto reduce the effective thickness of the epitaxial layer so as to inturn degrade the characteristics of the semiconductor device. For thisreason, it is necessary to suppress boron diffusion in the siliconsubstrate as much as possible.

It is known that diffusion of boron atoms contained in a siliconsubstrate is enhanced as the silicon substrate is subjected to a heattreatment in an oxidizing atmosphere. As thermal oxide film is formed onthe surface of a silicon substrate, some of the Si atoms in the siliconsubstrate are ousted from their places because their original places areoccupied by SiO₂ molecules and then pushed out from the crystal lattice.Then, the number of interstitial silicon atoms increases. On the otherhand, as boron atoms are replaced by interstitial silicon atoms, theyare diffused by kick-out diffusion. Thus, the diffusion of boron atomsis enhanced as a result of the increase in the number of interstitialsilicon atoms. Therefore, a technique of suppressing enhanced diffusionof boron atoms is to avoid as much as possible conducting any heattreatment in an oxidizing atmosphere.

However, there are instances where device characteristics are degradeddue to boron diffusion even when the device fabrication process does notinclude any heat treatment in an oxidizing atmosphere. Therefore, it iseagerly desired to improve the existing techniques for suppressingenhanced diffusion of boron atoms.

Means for Solving the Problems

In view of the above identified technical problems, it is therefore anobject of the present invention to provide an epitaxial silicon wafer inwhich enhanced boron diffusion in the silicon substrate is suppressed.

To solve the above problems, the inventors of the present inventionintensively looked into the mechanism of boron diffusion in siliconwafers. As a result of the intensive research efforts, the inventorshave made it clear that not only in heat treatments in an oxidizingatmosphere, but also in any heat treatments, interstitial silicon atomsare released as oxygen precipitates grows in the silicon substrate andkick-out diffusion of boron atoms is promoted by way of interstitialsilicon atoms. In particular, boron diffusion attributable to oxygenprecipitates rapidly progresses when the density of oxygen precipitatesin the silicon substrate exceeds a threshold level. The presentinvention is achieved by the inventors of the present invention as theypaid attention to this fact.

Thus, the present invention is based on the above-described technicalfindings. An epitaxial silicon wafer according to the present inventionis characterized in that an epitaxial layer is formed on a surface of aboron-doped silicon substrate, wherein the boron concentration in thesilicon substrate is 2.7×10¹⁷ atoms/cm³ or more and 1.3×10¹⁹ atoms/cm³or less, an initial oxygen concentration in the silicon substrate is11×10¹⁷ atoms/cm³ or less, and when an oxygen precipitate evaluationheat treatment is executed on the epitaxial silicon wafer, the densityof oxygen precipitates in the silicon substrate is 1×10¹⁰/cm³ or less.

Thus, according to the present invention, the density of oxygenprecipitates in the silicon substrate is 1×10¹⁰/cm³ or less and,therefore, even when oxygen precipitates in the silicon substrate growsas a result of any heat treatment in the device fabrication process, itis possible to suppress the increase in the number of interstitialsilicon atoms that arises as a result of a rise of the density of oxygenprecipitates. Then, it is also possible to suppress the kick-outdiffusion of boron atoms in the silicon substrate toward the epitaxiallayer side by way of the interstitial silicon atoms so that consequentlydiffusion of boron atoms can be suppressed to a level that ispractically equal to the level of boron diffusion that is observed whenthe density of oxygen precipitates is substantially nil.

In the present invention, the boron concentration in the siliconsubstrate is preferably 2.7×10¹⁷ atoms/cm³ or more and 1.3×10¹⁹atoms/cm³ or less and the initial oxygen concentration in the siliconsubstrate is preferably 11×10¹⁷ atoms/cm³ or less. When the initialoxygen concentration in the silicon substrate is 11×10¹⁷ atoms/cm³ orless, the density of oxygen precipitates in the silicon substrate can beheld to 1×10¹⁰/cm³ or less even when oxygen precipitates in the siliconsubstrate grows due to any heat treatment in the device fabricationprocess.

In the present invention, the boron concentration Y (atoms/cm³) and theinitial oxygen concentration X (×10¹⁷ atoms/cm³) preferably satisfy arelational expression of X≤−4.3×10⁻¹⁹Y+16.3. So long as the boronconcentration and the initial oxygen concentration in the siliconsubstrate satisfy the above defined requirement, the density of oxygenprecipitates can be held to 1×10¹⁰/cm³ or less regardless of the boronconcentration in the silicon substrate. Then, therefore, it is possibleto suppress enhanced boron diffusion attributable to oxygenprecipitates.

Advantages of the Invention

Thus, as described above, the present invention can provide an epitaxialsilicon wafer that can suppress enhanced boron diffusion in the siliconsubstrate even when oxygen precipitates grow due to any heat treatmentin the device fabrication process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a structure ofan epitaxial silicon wafer according to an embodiment of the presentinvention;

FIG. 2 is a flowchart illustrating a process of manufacturing theepitaxial silicon wafer;

FIG. 3 is a graph illustrating the depth profiling of boronconcentration of each of the epitaxial silicon wafer samples #1 through#4 before and after the oxygen precipitate evaluation heat treatment;and

FIG. 4 is a graph illustrating the relationship among the density ofoxygen precipitates, the initial oxygen concentration and the boronconcentration.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view illustrating a structure ofan epitaxial silicon wafer according to an embodiment of the presentinvention.

As shown in FIG. 1, an epitaxial silicon wafer 10 of this embodimentcomprises a silicon substrate 11 and an epitaxial layer 12 formed on thesurface of the silicon substrate 11. The silicon substrate 11 is apolished wafer that is cut out from a silicon monocrystalline ingotgrown by means of the Czochralski method (CZ method) and has amirror-polished surface. The silicon substrate 11 takes a role ofsecuring mechanical strength of the epitaxial silicon wafer 10 and, atthe same time, operating as gettering sink for capturing heavy metals.While the thickness of the silicon substrate 11 is not specificallydefined so long as it can secure mechanical strength of the epitaxialsilicon wafer 10, it may typically be made to be equal to 725 mm.

The silicon substrate 11 is a boron-doped p-type silicon substrate. Theboron concentration in the silicon substrate 11 is preferably 2.7×10¹⁷atoms/cm³ or more and 1.3×10¹⁹ atoms/cm³ or less while the electricalresistivity of the silicon substrate 11 is preferably 20 mΩ·cm or less.The electrical resistivity of the silicon substrate 11 can be reduced tosuch a low level and the silicon substrate 11 can be provided with asufficiently high gettering ability by using a silicon substrate 11 thatis doped with boron to such a high concentration level.

An epitaxial layer 12 is formed on the surface of the silicon substrate11. A semiconductor device such as a MOS transistor may typically beformed in the epitaxial layer 12. The thickness of the epitaxial layer12 is preferably between 1 and 10 μm. The epitaxial layer 12 may have amultilayer structure formed by laying a plurality of epitaxial layershaving different characteristics one on the other. Normally, theelectrical resistivity of the epitaxial layer 12 is made to be higherthan the electrical resistivity of the silicon substrate 11 and thesilicon substrate is made to contain a p-type dopant (boron) or ann-type dopant (phosphor, arsenic or antimony) added thereto.

When an oxygen precipitate evaluation heat treatment is executed on theepitaxial silicon wafer 10, the density of oxygen precipitates in thesilicon substrate 11 is 1×10¹⁰/cm³ or less. As will be described ingreater detail hereinafter, the oxygen precipitates in the siliconsubstrate 11 will only minimally influence the boron diffusion when thedensity of oxygen precipitates is 1×10¹⁰/cm³ or less. Then, the borondiffusion can be held to a level that is practically equal to thediffusion level that is observed when the density of oxygen precipitatesis substantially nil.

An oxygen precipitate evaluation heat treatment is a two-step heattreatment in which typically a first heat treatment (core forming step)and a second heat treatment (core growing step) are sequentiallyconducted respectively at 700° C. for 3 hours and at 1,000° C. for 16hours. They are heat treatments simulating those of a device fabricationprocess. The heat treatments are conducted not in an oxidizingatmosphere but in a nitrogen atmosphere so that no thermal oxide film isformed there and hence there arises no problem of enhanced diffusion ofboron atoms attributable to production of thermal oxide film. However,as the oxygen precipitation core grows in the silicon substrate 11, thedensity of oxygen precipitates rises to give rise to an additional causeof boron diffusion. Furthermore, it is known that boron in the siliconsubstrate 11 exerts an effect of promoting oxygen precipitation andtherefore, as the boron concentration in the silicon substrate 11 rises,the density of oxygen precipitates in the silicon substrate 11 alsorises. While the existence of oxygen precipitates is necessary to someextent for the purpose of securing the gettering ability, enhanced borondiffusion occurs as the volume of oxygen precipitate increases.

To make the density of oxygen precipitate in the silicon substrate 111×10¹⁰/cm³ or less, it is necessary to make the interstitial initialoxygen concentration of the silicon substrate 11 11×10¹⁷ atoms/cm³ orless. If the initial oxygen concentration is higher than 11×10¹⁷atoms/cm³, the number of interstitial silicon atoms rises as the densityof oxygen precipitate increases to consequently allow enhanced borondiffusion to occur by way of interstitial silicon atoms. The lower theinitial oxygen density, the better for the density of oxygen precipitateto be held below the above defined limit level. While no lower limit isdefined for the initial oxygen concentration, it is currently impossibleto grow silicon single crystals having an initial oxygen concentrationlower than 11×10¹⁷ atoms/cm³ from the viewpoint of actual manufacturing.Note here that all the oxygen concentrations cited in this patentspecification are measured values obtained by means of Fourier transforminfrared spectrometry (FT-IR) as defined in ASTM F-121 (1979).

The preferred range of initial oxygen concentration in the siliconsubstrate 11 changes as a function of the boron concentration in thesubstrate. When the boron concentration in the silicon substrate 11 islow, no problem arises if the initial oxygen concentration is high tosome extent. However, when the boron concentration is high, oxygenprecipitate is produced excessively unless the initial oxygenconcentration is held low because oxygen precipitate can easily grow insuch an environment so that it may not be possible to make the densityof oxygen precipitate 1×10¹⁰/cm³ or less. When the initial oxygenconcentration in the silicon substrate 11 is expressed by X (×10¹⁷atoms/cm³) and the boron concentration is expressed by Y (atoms/cm³),they preferably satisfy the requirement of the relational expression ofX≤−4.3×10⁻¹⁹Y+16.3. As long as they satisfy the requirement of the abovedefined relational expression, the density of oxygen precipitate in thesilicon substrate 11 can be held to be 1×10¹⁰/cm³ or less regardless ofthe boron concentration.

FIG. 2 is a flowchart illustrating a process of manufacturing theepitaxial silicon wafer 10.

As shown in FIG. 2, to manufacture the epitaxial silicon wafer 10,firstly a boron-doped silicon single crystal ingot is made to grow byway of the Cz process (Step S1). In this step, the silicon singlecrystal is doped with boron to a concentration level between 2.7×10¹⁷atoms/cm³ and 1.3×10¹⁹ atoms/cm³. Although the silicon single crystalcontains oxygen to the level of super saturation because oxygen iseluted from the quartz crucible that is being employed for themanufacturing process, the oxygen concentration in the silicon singlecrystal can be controlled by controlling the single crystal pull-upconditions. More specifically, the single crystal pull-up conditions arecontrolled such that the initial oxygen concentration X (×10¹⁷atoms/cm³) and the boron concentration Y (atoms/cm³) in the siliconsingle crystal satisfy the above-described requirement of the relationalexpression of X≤−4.3×10⁻¹⁹Y+16.3.

When silicon is put into a quartz crucible as starting material, apredetermined amount of boron is added to the raw material so as to makethe single crystal pulled up from the quartz crucible contain boron.More specifically, boron is added by the amount that makes the singlecrystal show the intended electrical resistivity at the top position ofthe single crystal. Then, as the added boron is molten with the startingmaterial of silicon, boron-containing silicon melt is produced. Whilethe single crystal that is pulled up from the silicon melt containsboron at a given content ratio, the boron concentration rises in theingot pull-up direction as the crystal growth progresses due tosegregation. Therefore, the oxygen concentration needs to be reduced inthe ingot pull-up direction so as to make the ingot satisfy therequirement of the above defined relational expression.

The oxygen concentration in the single crystal can be controlled byadjusting the rotational speed of the quartz crucible and/or the powersupplied to the heater. To reduce the oxygen concentration in the singlecrystal, it is sufficient to select a low rotational speed for thequartz crucible and/or a low power output for the heater. Thus, theoxygen concentration in the single crystal can be held low bycontrolling the conditions under which the single crystal is pulled upin the above-described manner.

The MCZ method of pulling up the single crystal, while applying amagnetic field to the silicon melt, is very effective for reducing theoxygen concentration in the pulled up single crystal. With the MCZmethod, the convection of the silicon melt can be suppressed under theinfluence of the magnetic field so that consequently the elution ofoxygen from the quartz crucible into the silicon melt can be suppressedand hence the oxygen concentration in the single crystal that is pulledup from the silicon melt can also be held to a low level.

Then, the silicon single crystal ingot is processed to produce thesilicon substrate 11 (Step S2). As described above, the siliconsubstrate 11 is a polished wafer that is cut out from the silicon singlecrystal ingot and whose surface is mirror-polished. The boronconcentration of the silicon substrate 11 is 2.7×10¹⁷ atoms/cm³ or moreand 1.3×10¹⁹ atoms/cm³ or less and the initial oxygen concentration inthe silicon substrate 11 is 11×10¹⁷ atoms/cm³ or less.

Next, an epitaxial layer 12 is formed on the surface of the siliconsubstrate 11 by a well-known method (Step S3). As a result of executingthe above steps, a finished epitaxial wafer 10 is produced.

An epitaxial silicon wafer 10 that is manufactured in theabove-described way is then employed as substrate material forsemiconductor devices. Then, various semiconductor devices can beproduced by using such an epitaxial silicon wafer by way of variousprocessing steps. Such processing steps include various heat treatmentsteps and, as a result, an oxygen precipitation core is formed in thesilicon substrate 11, which oxygen precipitation core grows to increasethe density of oxygen precipitate in the silicon substrate. However,since the density of oxygen precipitate in the silicon substrate is heldto be 1×10¹⁰/cm³ or less, any enhanced boron diffusion attributable tooxygen precipitate can be prevented from taking place.

If the boron concentration and the initial oxygen concentration in thesilicon substrate are known and the heat treatment conditions (heathistory) in the device fabrication process are also known, the densityof oxygen precipitate and the extent of enhanced boron diffusion in thesilicon substrate that will be observed when such an epitaxial siliconwafer is heat treated during the device fabrication process can bepredicted by means of simulation. If, as a result of such simulation,the width of the transition region that is broadened by enhanced borondiffusion cannot be confined within the given permissible range, it mayonly be necessary to adjust the initial oxygen concentration so as toconfine the width of the transition region within the given permissiblerange. Thus, the initial oxygen concentration in the silicon substratethat is necessary to produce a given density of oxygen precipitate canbe predicted from the heat treatment conditions in the devicefabrication process, so that the enhanced boron diffusion can beconfined within a given permissible range.

As described above in detail, the epitaxial silicon wafer 10 of thisembodiment comprises a boron-doped silicon substrate 11 and an epitaxiallayer 12 formed on the surface of the silicon substrate 11 and, when anoxygen precipitate evaluation heat treatment is conducted, the densityof oxygen precipitates in the silicon substrate 11 is found to be1×10¹⁰/cm³ or less. Thus, this embodiment can suppress any enhanceddiffusion of boron that can take place from the silicon substrate 11toward the epitaxial layer 12 as a result of a rise in the density ofoxygen precipitates.

The epitaxial silicon wafer 10 of this embodiment can advantageously beemployed as substrate material for a backside illumination typesolid-state imaging element. In the process of manufacturing a backsideillumination type solid-state imaging element, metal impuritiescontained in the silicon substrate can increase the dark current of thesensor section to in turn give rise to defects that are referred to aswhite flaws. However, the use of a p-type silicon substrate that isdoped with boron atoms to a high concentration level can solve theproblem of metal impurities because the silicon substrate operates asgettering sink.

Additionally, in a backside illumination type solid-state imagingelement, the wiring layer and related parts are arranged in layerslocated lower than the sensor section so that the sensor section candirectly take in light coming from outside. Then, as a result, theimaging element can produce clear images including moving images. It isnecessary to execute a process of removing the silicon substrate 11typically by polishing in order to arrange the wiring layer and relatedparts in layers located lower than the sensor section and causing onlythe epitaxial layer 12 to be left undamaged. If the transition region inthe epitaxial layer is broadened by enhanced boron diffusion to degradethe uniformity of in-plane electrical resistivity of the wafer, itbecomes difficult to determine the appropriate extent to which thesilicon substrate 11 is to be polished and, additionally, thecharacteristics of the solid-state imaging element can be degradedbecause the effective thickness of the epitaxial layer 12 can bereduced. However, the above identified problems will be dissolved tomake it possible to manufacture a high quality backside illuminationtype imaging element when the width of the transition region issatisfactorily narrow and the effective thickness of the epitaxial layer12 is sufficiently large.

While preferred embodiments of the present invention have been explainedabove, the present invention is not limited thereto. Variousmodifications can be made to the embodiments without departing from thescope of the present invention and it is needless to say that suchmodifications are also embraced within the scope of the invention.

Examples

A silicon substrate with (100) crystal plane orientation was cut outfrom a silicon single crystal ingot grown by means of the CZ method andthe surface of the silicon substrate was mirror-polished. The siliconsubstrate contained boron added thereto to a concentration of 1.0×10¹⁹atoms/cm³. The initial oxygen concentration of the silicon substrate was6×10¹⁷ atoms/cm³. An epitaxial layer of 5 μm thickness was formed on thesurface of the silicon substrate by vapor deposition at a temperature of1150° C. to obtain a sample of epitaxial silicon wafer, which will bereferred to as epitaxial silicon wafer Sample #1 hereinafter.Additionally, epitaxial silicon wafer samples of Samples #2 through #4,which showed respective initial oxygen concentrations that differed fromthe initial oxygen concentration of Sample #1, were prepared by way ofrespective processes similar to the process of preparing Sample #1. Theinitial oxygen concentration of Sample #2, that of Sample #3 and that ofSample #4 were respectively 10×10¹⁷ atoms/cm³, 11×10¹⁷ atoms/cm³ and13×10¹⁷ atoms/cm³.

Then, an oxygen precipitate evaluation heat treatment was conducted oneach of the epitaxial silicon wafer samples #1 through #4. In each ofthe oxygen precipitate evaluation heat treatments, a heat treatment wasconducted at 700° C. in a nitrogen atmosphere for 3 hours andsubsequently another heat treatment was conducted at 1,000° C. also inan nitrogen atmosphere for 16 hours. Additionally, depth profiling ofboron concentration of each of the samples #1 through #4 was observed bySIMS (secondary ion mass spectroscopy) before and after the oxygenprecipitate evaluation heat treatment.

FIG. 3 is a graph illustrating the depth profiling of boronconcentration of each of the epitaxial silicon wafer samples #1 through#4 before and after the oxygen precipitate evaluation heat treatment.The horizontal axis of the graph indicates the depth (relative value)from the uppermost surface of the wafer and the vertical axis of thegraph indicates the boron concentration (relative value).

As shown in FIG. 3, all of the epitaxial silicon wafer samples #1through #4 before the oxygen precipitate evaluation heat treatmentsshowed boron concentration profiles that are substantially the same witheach other. Each of them showed an abrupt change at and near theboundary of the silicon substrate and the epitaxial layer to prove thatthe amount of boron that diffused into the epitaxial layer there wasvery small. The broken line X having long segments commonly shows theboron concentration profiles of the samples #1 through #4 before theoxygen precipitate evaluation heat treatments.

On the other hand, all the boron concentration profiles of the samples#1 through #4 changed remarkably after the respective oxygen precipitateevaluation heat treatments if compared with the concentration profilesthat were observed before the evaluation heat treatments. Morespecifically, the boron diffusion into the epitaxial layer increased toa large extent after the oxygen precipitate evaluation heat treatment ineach of the samples. Thus, it may be safe to assume that the thermaldiffusion due to the evaluation heat treatment is mainly responsible forthe remarkable change in the boron concentration profile of each of thesamples.

Out of the samples, the boron concentration profile (solid line) of“Sample #1” showed that practically no boron atoms existed at and nearthe surface of the epitaxial layer. In other words, Sample #1 showed anexcellent result. The boron concentration abruptly increased at aboutdepth 0.7. The boron concentration was 0.015 at depth 0.8, 0.2 at depth0.9 and 0.5 at depth 1. Both “Sample #2” (broken line having shortsegments) and “Sample #3” (dotted line) showed respective boronconcentration profiles that were substantially the same as that ofSample #1.

The boron concentration profile (dashed dotted line) of “Sample #4”largely differed from the boron concentration profiles of Samples #1through #3″. Boron diffusion progressed to near the surface of theepitaxial layer in Sample #4. More specifically, the boron concentrationstarted to increase at about depth 0.6 and the boron concentrations atdepth 0.7, at depth 0.8 and at depth 0.9 were respectively 0.004, 0.07and 0.25. The boron concentration at depth 1 was 0.5, which was the sameas the boron concentration at depth 1 of each of Samples #1 through #3.

From the obtained results as described above it became clear that borondiffusion was scarcely observed in Samples #1 through #3 but borondiffused very remarkably in Sample #4. It may be safe to assume thatenhanced boron diffusion was mainly responsible for the phenomenon thatthe boron concentration profile of Sample #4 changed particularlyremarkably.

Then, after the oxygen precipitate evaluation heat treatment, each ofthe epitaxial silicon wafer samples #1 through #4 were cleaved in thethickness direction and the cleaved cross-section of each of them wassubjected to a selective etching process of etching it to a thickness of2 μm by means of a wright etching solution. Subsequently, the centralpart of the cleaved cross-section in the thickness direction of thesilicon wafer was observed through an optical microscope and the numberof etch pits within the 100 μm×100 μm square area was measured asdensity of oxygen precipitates. Table 1 below shows the obtainedresults.

TABLE 1 Density of Initial oxygen Boron oxygen Wafer concentrationconcentration precipitates sample (atoms/cm³) (atoms/cm³) (/cm³) #1  6 ×10¹⁷ 1.0 × 10¹⁹ <1 × 10⁷   #2 10 × 10¹⁷ 1.0 × 10¹⁹ 1 × 10⁹  #3 11 × 10¹⁷1.0 × 10¹⁹ 1 × 10¹⁰ #4 13 × 10¹⁷ 1.0 × 10¹⁹ 3 × 10¹⁰

As seen from Table 1, the density of oxygen precipitates of Sample #1was below the measurement threshold (less than 1×10⁷/cm³). The densityof oxygen precipitates of Sample #2 and that of Sample #3 wererespectively 1×10⁹/cm³ and 1×10¹⁰/cm³, whereas that of Sample #4 was3×10¹⁰/cm³. From the results shown in Table 1 and the graph of FIG. 3,it became clear that enhanced boron diffusion was scarcely observed ineach of Samples #1 through #3, whose density of oxygen precipitates was1×10¹⁰/cm³ or less.

Thereafter, a total of 28 samples of epitaxial silicon wafer that weredifferentiated by using the initial oxygen concentration and the boronconcentration in the silicon substrate as parameters were prepared inorder to look into the mutual relationship among the initial oxygenconcentration, the boron concentration and the density of oxygenprecipitates and each of the samples was subjected to an oxygenprecipitate evaluation heat treatment and subsequently the density ofoxygen precipitates thereof was measured.

FIG. 4 is a graph illustrating the relationship among the density ofoxygen precipitates, the initial oxygen concentration and the boronconcentration. The horizontal axis of the graph indicates the oxygenconcentration (×10¹⁷ atoms/cm³) and the vertical axis indicates theboron concentration (atoms/cm³). A “o” mark was plotted for a sampleshowing a density of oxygen precipitates that was 1×10¹⁰/cm³ or less,whereas a “x” mark was plotted for a sample showing a density of oxygenprecipitates that was more than 1×10¹⁰/cm³.

As clearly seen from FIG. 4, it was found that the requirement ofdensity of oxygen precipitates of 1×10¹⁰/cm³ or less cannot be metunless the initial oxygen concentration is made low when the boronconcentration is high. For example, when the boron concentration was aslow as 4.8×10¹⁸ atoms/cm³, the highest value of the initial oxygenconcentration that could meet the requirement of density of oxygenprecipitates of 1×10¹⁰/cm³ or less was about 14×10¹⁷ atoms/cm³. When, onthe other hand, the boron concentration was as high as 1.6×10¹⁹atoms/cm³, the highest value of the initial oxygen concentration thatcould meet the requirement of density of oxygen precipitates of1×10¹⁰/cm³ or less was about 9×10¹⁷ atoms/cm³.

The boundary line separating the plotted “0” marks and the plotted “x”marks was expressed by means of a linear function to define the regionof the plotted “o” marks. From the obtained results as described above,it became clear that the density of oxygen precipitates can be made tobe 1×10¹⁰/cm³ or less when the oxygen concentration X (×10¹⁷ atoms/cm³)and the boron concentration Y (atoms/cm³) satisfy the requirement ofX≤−4.3×10⁻¹⁹Y+16.3.

REFERENCE SIGNS LIST

-   10 epitaxial silicon wafer-   11 silicon substrate-   12 epitaxial layer

What is claimed is:
 1. An epitaxial silicon wafer in which an epitaxial layer is formed on a surface of a boron-doped silicon substrate, wherein the boron concentration in the silicon substrate is 2.7×10¹⁷ atoms/cm³ or more and 1.3×10¹⁹ atoms/cm³ or less, an initial oxygen concentration in the silicon substrate is 11×10¹⁷ atoms/cm³ or less, and when an oxygen precipitate evaluation heat treatment is executed on the epitaxial silicon wafer, the density of oxygen precipitate in the silicon substrate is 1×10¹⁰/cm³ or less.
 2. The epitaxial silicon wafer as claimed in claim 1, wherein the boron concentration Y (atoms/cm³) and the initial oxygen concentration X (×10¹⁷ atoms/cm³) satisfy a relational expression of X≤−4.3×10⁻¹⁹Y+16.3. 